1. Field of the Invention
This invention relates to the testing of underground formations or reservoirs. More particularly, this invention relates to a method and apparatus for isolating a downhole reservoir, and testing the reservoir formation and fluid.
2. Background
While drilling a well for commercial development of hydrocarbon reserves, several subterranean reservoirs and formations are encountered. In order to discover information about the formations, such as whether the reservoirs contain hydrocarbons, logging devices have been incorporated into drill strings to evaluate several characteristics of these reservoirs. Measurement-while-drilling systems (hereinafter MWD) have been developed that contain resistivity, nuclear and other logging devices which can constantly monitor formation and reservoir characteristics during drilling of wellbores. The MWD systems can generate data that includes information about the presence of hydrocarbon presence, saturation levels, and formation porosity. Telemetry systems have been developed for use with the MWD systems to transmit the data to the surface. A common telemetry method is the mud-pulsed system, an example of which is found in U.S. Pat. No. 4,733,233. MWD systems provide real time analysis of the subterranean reservoirs.
Commercial development of -hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. Despite the advances in data acquisition during drilling, using the MWD systems, it is often necessary to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after the well has been drilled, the hydrocarbon zones are often tested by other test equipment.
One type of post-drilling test involves producing fluid from the reservoir, collecting samples, shutting-in the well and allowing the pressure to build-up to a static level. This sequence may be repeated several times for different reservoirs within a given borehole. This type of test is known as a xe2x80x9cPressure Build-up Testxe2x80x9d. One of the important aspects of the data collected during such a test is the pressure build-up information gathered after drawing the pressure down. From this data, information can be derived as to permeability, and size of the reservoir. Further, actual samples of the reservoir fluid are obtained, and tested to gather Pressure-Volume-Temperature data relevant to the reservoir""s hydrocarbon distribution.
In order to perform these important tests, it is currently necessary to retrieve the drill string from the well borehole. Thereafter, a different tool, designed for the testing, is run into the well borehole. A wireline is often used to lower a test tool into the well borehole. The test tool sometimes utilizes packers for isolating the reservoir. Numerous communication devices have been designed which provide for manipulation of the test tool, or alternatively, provide for data transmission from the test tool. Some of those designs include signaling from the surface of the Earth with pressure pulses, through the fluid in the well borehole, to or from a downhole microprocessor located within, or associated with the test tool. Alternatively, a wire line can be lowered from the surface, into a landing receptacle located within a test tool, establishing electrical signal communication between the surface and the test assembly. Regardless of the type of test tool and type of communication system used, the amount of time and money required for retrieving the drill string and running a second test tool into the borehole is significant. Further, if the borehole is highly deviated, a wire line tool is difficult to use to perform the testing.
There is also another type of problem, related to downhole pressure conditions, which can occur during drilling. The density of the drilling fluid is calculated to achieve maximum drilling efficiency while maintaining safety, and the density is dependent upon the desired relationship between the weight of the drilling mud column and the downhole pressures which will be encountered. As different formations are penetrated during drilling, the downhole pressures can change significantly. Currently available devices do not accurately sense the formation pressure as the drill bit penetrates the formation. The actual formation pressure could be lower than expected, allowing the lowering of mud density, or the formation pressure could be higher than expected, possibly even resulting in a pressure kick Consequently, since this information is not easily available to the operator, the drilling mud may be maintained at too high or too. low a density for maximum efficiency and maximum safety.
Therefore, there is a need for a method and apparatus that will allow for the pressure testing and fluid sampling of potential hydrocarbon reservoirs as soon as the borehole has been drilled into the reservoir, without removal of the drill string. Further, there is a need for a method and apparatus that will allow for adjusting drilling fluid density in response to changes in downhole pressures to achieve maximum drilling efficiency. Finally, there is a need for a method and apparatus that will allow for blow out prevention downhole, to promote drilling safety.
A formation testing method and a test apparatus are disclosed. The test apparatus is mounted on a work string for use in a well borehole filled with fluid. It can be a work string designed for drilling, re-entry work, or workover applications. As required for many of these applications, the work string may be one capable of going into highly deviated holes, horizontally, or even uphill. Therefore, in order to be fully useful to accomplish the purposes of the present invention, the work string must be one that is capable of being forced into the hole, rather than being dropped like a wireline. The work string can contain a Measurement While Drilling (MWD) system and a drill bit, or other operative elements. The formation test apparatus may include at least one expandable packer or other extendable structure that can expand or extend to contact the wall of the well borehole; device for moving fluid such as a pump, for taking in formation -fluid; a non-rotating sleeve; an extendable stabilizer blade; a coring device, and at least one sensor for measuring a characteristic of the fluid or the formation. The test apparatus will also contain a controller, for controlling the various valves or pumps which are used to control fluid flow. The sensors and other instrumentation and control equipment must be carried by the tool. The tool must have a communication system capable of communicating with the surface, and data can be telemetered to the surface or stored in a downhole memory for later retrieval.
The method involves drilling or re-entering a borehole and selecting an appropriate underground reservoir. The pressure, or some other characteristic of the fluid in the well borehole at the reservoir, the rock, or both, can then be measured. The extendable element, such as a packer or test probe, is set against the wall of the borehole to isolate a portion of the borehole or at least a portion of the borehole wall. In the non-rotatable sleeve embodiment, the drill string can continue rotating and advancing while the sleeve is held stationary during performance of the test.
If two packers are used, this will create an upper annulus, a lower annulus, and an intermediate annulus within the well borehole. The intermediate annulus corresponds to the isolated portion of the borehole, and it is positioned at the reservoir to be tested. Next, the pressure, or other property, within the intermediate annulus is measured. The well borehole fluid, primarily-drilling-mud, may then be withdrawn from the intermediate annulus with the pump. The level at which pressure within the intermediate annulus stabilizes may then be measured; it will correspond to the formation pressure. Pressure can also be applied to fracture the formation, or to perform a pressure test of the formation. Additional extendable elements may also be provided, to isolate two or more permeable zones. This allows the pumping of fluid from one or more zones to one or more other zones.
Alternatively, a piston or other test probe can be extended from the test apparatus to contact the borehole wall in a sealing relationship, or some other expandable element can be extended to create a zone from which essentially pristine formation fluid can be withdrawn. Further, the extendable probe can be used to position a sensor directly against the borehole wall, for analysis of the formation, such as by spectroscopy. Extension of the probe could also be accomplished by extending a locating arm or stabilizer rib from one side of the test tool, to force the opposite side of the test tool to contact the borehole wall, thereby exposing a sample port to the formation fluid. Regardless of the apparatus used, the goal is to establish a zone of pristine formation fluid from which a fluid or core sample can be taken, or in which characteristics of the fluid can be measured. This can be accomplished by various embodiments. The example first mentioned above is to use inflatable packers to isolate a portion of the entire borehole, subsequently withdrawing drilling fluid from the isolated portion until it fills with formation fluid. The other examples given accomplish the goal by expanding an element against a spot on the borehole wall, thereby directly contacting the formation and excluding drilling fluid.
The apparatus should be constructed so as to be protected during performance of the primary operations for which the work string is intended, such as drilling, re-entry, or workover. If an extendable probe is used, it can retract within the tool, or it can be protected by adjacent stabilizers, or both. A packer or other extendable elastomeric element can retract within a recess in the tool, or it can be protected by a sleeve or some other type of cover.
In addition to the pressure sensor mentioned above, the formation test apparatus can contain a resistivity sensor for measuring the resistivity of the well borehole fluid and the formation fluid, or other types of sensors. The resistivity of the drilling fluid is usually noticeably different from the resistivity of the formation fluid. If two packers are used, the resistivity of fluid being pumped from the intermediate annulus can be monitored to determine when all of the drilling fluid has been withdrawn from the intermediate annulus. As flow is induced from the isolated formation into the intermediate annulus, the resistivity of the fluid being pumped from the intermediate annulus is monitored. Once the resistivity of the exiting fluid differs sufficiently from the resistivity of the well borehole fluid, it is assumed that formation fluid has filled the intermediate annulus, and the flow is terminated. This can also be used to verify a proper seal of the packers, since leaking of drilling fluid past the packers would tend to maintain the resistivity at the level of the drilling fluid. Other types of sensors which can be incorporated are flow rate measuring devices, viscosity sensors, density measuring devices,- dielectric property measuring devices, and optical spectroscopes.
After shutting in the formation, the pressure in the intermediate annulus can be monitored. Pumping can also be resumed, to withdraw formation fluid from the intermediate annulus at a measured rate. Pumping of formation fluid and measurement of pressure can be sequenced -as desired to provide data which can be used to calculate various properties of the formation, such as permeability and size. If direct contact with the borehole wall is used, rather than isolating a section of the borehole, similar tests can be performed by incorporating test chambers within the test apparatus. The test chambers can be maintained at atmospheric pressure while the work string is being drilled or lowered into the borehole. Then, when the extendable element has been placed in contact with the formation, exposing a test port to the formation fluid, a test chamber can be selectively placed in fluid communication with the test port. Since the formation fluid will be at much higher pressure than atmospheric, the formation fluid will flow into the test chamber. In this way, several test chambers can be used to perform different pressure tests or take fluid samples.
In some embodiments which use expandable packers, the formation test apparatus has contained therein a drilling fluid return flow passageway for allowing return flow of the drilling fluid from the lower annulus to the upper annulus. Also included is at least one pump, which can be a Venturi pump or any other suitable type of pump, for preventing overpressurization in an intermediate annulus. Overpressurization can be undesirable because of the possible loss of the packer seal, or because it can hamper operation of extendable elements which may be operated by differential pressure between the inner bore of the work string and the annulus, or by a fluid pump. To prevent overpressurization, the drilling fluid is pumped down the longitudinal. inner bore of the work string, past the lower end of the work string (which is generally the bit), and up the annulus. Then the fluid is channeled through return flow passageway and the Venturi pump, creating a low pressure zone at the Venturi, so that the fluid within the intermediate annulus is held at a lower pressure than the fluid in the return flow passageway.
The device may also include a circulation valve, for opening and closing the inner bore of the work string. A shunt valve can be located in the work string and operatively associated with the circulation valve, for allowing flow from the inner bore of the work string to the annulus around the work string, when the circulation valve is closed. These valves can be used in operating the test apparatus as a down hole blow-out preventor.
In most embodiments, one or more gripper elements may be incorporated on the work string or non-rotating sleeve. The grippers are extendable and are used to engage the borehole well. Once the borehole wall is engaged, the grippers anchor the work string or non-rotating sleeve such that the work string or non-rotating sleeve remains substantially motionless during a test. The advantage of anchoring the tool is increased useful life of soft components such as pad members and packers.
In the case where an influx of reservoir fluids invade the borehole, which is sometimes referred to as a xe2x80x9ckickxe2x80x9d, the method includes the steps of setting the expandable packers, and then positioning the circulating valve in the closed position. The packers are set at a position that is above the influx zone so that the influx zone is isolated. Next, the shunt valve is placed in the open position. Additives can then be added to the drilling fluid, thereby increasing the density of the mud. The heavier mud is circulated down the work string, through the shunt valve, to fill the annulus. Once the circulation of the denser drilling fluid is completed, the packers can be unseated and the circulation valve can be opened. Drilling may then resume.
An advantage of the present invention includes use of the pressure and resistivity sensors with the MWD system, to allow for real time data transmission of those measurements. Another advantage is that the present invention allows obtaining static pressures, pressure build-ups, and pressure draw-downs with the work string, such as a drill string, in place. Computation of permeability and other reservoir parameters based on the pressure measurements can be accomplished without pulling the drill string.
The packers can be set multiple times, so that testing of several zones is possible. By making-measurement of the down hole conditions possible -in real time, optimum drilling fluid conditions can be determined which will aid in hole cleaning, drilling safety, and drilling speed. When an influx of reservoir fluid and gas enter the well borehole, the high pressure is contained within the lower part of the well borehole, significantly reducing risk of being exposed to these pressures at surface. Also, by shutting-in the well borehole immediately above the critical zone, the volume of the influx into the well borehole is significantly reduced.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description in which similar reference characters refer to similar parts, and in which: